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Habitable Zone Planet Orbiting White Dwarf Star Discovered

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New research led by UCL scientists found a ring of planetary debris studded with moon-sized bodies orbiting close to an ultra-cool white dwarf star in the “habitable zone” where life and water could exist.

White dwarfs are stars that have burned through all of their hydrogen fuel and are now blazing embers. Almost all stars, including the Sun, will turn into white dwarfs at some point, but little is known about their planetary systems.

A multinational team of researchers used data from ground- and space-based observatories to detect light from a white dwarf in the Milky Way known as WD1054–226.

Surprisingly, they discovered strong dips in light every 25 hours, corresponding to 65 uniformly spaced clouds of planetary debris around the star. The researchers found that the transiting structures’ exact regularity – darkening the star’s light every 23 minutes – indicates that they are held in place by a nearby planet.

“This is the first time astronomers have detected any kind of planetary body in the habitable zone of a white dwarf,” says Professor Jay Farihi, lead author.

“The moon-sized structures we have observed are irregular and dusty (e.g. comet-like) rather than solid, spherical bodies.  Their absolute regularity, one passing in front of the star every 23 minutes, is a mystery we cannot currently explain.

“An exciting possibility is that these bodies are kept in such an evenly-spaced orbital pattern because of the gravitational influence of a nearby planet. Without this influence, friction and collisions would cause the structures to disperse, losing the precise regularity that is observed.  A precedent for this ‘shepherding’ is the way the gravitational pull of moons around Neptune and Saturn help to create stable ring structures orbiting these planets.

“The possibility of a planet in the habitable zone is exciting and also unexpected; we were not looking for this.  However, it is important to keep in mind that more evidence is necessary to confirm the presence of a planet. We cannot observe the planet directly so confirmation may come by comparing computer models with further observations of the star and orbiting debris.” 

This orbit around the white dwarf is thought to have been cleared during the massive star phase of its life, thus any planet capable of hosting water and consequently life would be a recent development. For at least two billion years, including at least one billion years into the future, the area would be habitable.

Habitable Zone Planet Orbiting White Dwarf Star Discovered
Habitable Zone Planet Orbiting White Dwarf Star Discovered

Over 95% of all stars will eventually turn into white dwarfs. The largest stars, which explode and become black holes or neutron stars, are the exceptions.

The lead author adds: “Since our Sun will become a white dwarf in a few billion years, our study provides a glimpse into the future of our own solar system.”

When a star runs out of hydrogen, it expands and cools, turning into a red giant. In four to five billion years, the Sun will enter this phase, swallowing Mercury, Venus, and possibly Earth. The hot core of the star remains after the outer material has softly blown away and hydrogen has been spent, slowly cooling over billions of years — this is the star’s white dwarf phase.

Astronomers have a hard time detecting planets around white dwarfs because the stars are much fainter than main-sequence stars (like the Sun). Astronomers have only discovered circumstantial evidence of a gas giant (similar to Jupiter) around a white dwarf so far.

Researchers used the ULTRACAM high-speed camera mounted on the ESO 3.5m New Technology Telescope (NTT) at the La Silla Observatory in Chile to observe WD1054–226, a white dwarf 117 light-years away, over the course of 18 nights to capture changes in its luminosity. The researchers used data from NASA’s Transiting Exoplanet Survey Satellite (TESS) to better understand the shifts in light, which allowed them to conclude the planetary structures had a 25-hour orbit.

They discovered that the light from WD1054–226 was always veiled by massive clouds of orbital material passing in front of it, implying that the star is surrounded by a ring of planetary debris.

The habitable zone, often known as the Goldilocks zone, is the temperature range in which liquid water might hypothetically exist on a planet’s surface. The habitable zone of a white dwarf will be smaller and closer to the star than that of a star like the Sun because white dwarfs emit less light and consequently heat.

The objects seen in the study orbit in a region that would have been engulfed by the star as a red giant, thus they are more likely to have originated or arrived later than to have survived the star’s and planetary system’s birth.

Source: 10.1093/mnras/stab3475

Image Credit: Mark A. Garlick / markgarlick.com

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